Enable GraphiQL UI by default with Devtools

GraphiQL is useful when working on a GraphQL API - it allows
developers to craft queries and test newly developed feature. This
is not enabled by default, as this should not be exposed in production
without the developer's knowledge.

This commit flips this value when Spring Boot devtools is active for a
better developer experience.

See gh-29140

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/env/DevToolsPropertyDefaultsPostProcessor.java

@@ -63,6 +63,7 @@ public class DevToolsPropertyDefaultsPostProcessor implements EnvironmentPostPro
 		Map<String, Object> properties = new HashMap<>();
 		properties.put("spring.thymeleaf.cache", "false");
 		properties.put("spring.freemarker.cache", "false");
+		properties.put("spring.graphql.graphiql.enabled", "true");
 		properties.put("spring.groovy.template.cache", "false");
 		properties.put("spring.mustache.cache", "false");
 		properties.put("server.servlet.session.persistent", "true");